Pure Appl. Geophys. 170 (2013), 299–318 2012 Springer Basel AG DOI 10.1007/s00024-012-0525-1
Pure and Applied Geophysics
Seismic Site Classification and Correlation between Standard Penetration Test N Value and Shear Wave Velocity for Lucknow City in Indo-Gangetic Basin P. ANBAZHAGAN,1 ABHISHEK KUMAR,1 and T. G. SITHARAM1 Abstract—Subsurface lithology and seismic site classification of Lucknow urban center located in the central part of the IndoGangetic Basin (IGB) are presented based on detailed shallow subsurface investigations and borehole analysis. These are done by carrying out 47 seismic surface wave tests using multichannel analysis of surface waves (MASW) and 23 boreholes drilled up to 30 m with standard penetration test (SPT) N values. Subsurface lithology profiles drawn from the drilled boreholes show low- to medium-compressibility clay and silty to poorly graded sand available till depth of 30 m. In addition, deeper boreholes (depth [150 m) were collected from the Lucknow Jal Nigam (Water Corporation), Government of Uttar Pradesh to understand deeper subsoil stratification. Deeper boreholes in this paper refer to those with depth over 150 m. These reports show the presence of clay mix with sand and Kankar at some locations till a depth of 150 m, followed by layers of sand, clay, and Kankar up to 400 m. Based on the available details, shallow and deeper cross-sections through Lucknow are presented. Shear wave velocity (SWV) and N-SPT values were measured for the study area using MASW and SPT testing. Measured SWV and N-SPT values for the same locations were found to be comparable. These values were used to estimate 30 m average values of N-SPT (N30) and SWV (V30 s ) for seismic site classification of the study area as per the National Earthquake Hazards Reduction Program (NEHRP) soil classification system. Based on the NEHRP classification, the entire study area is classified into site class C and D based on V30 s and site class D and E based on N30. The issue of larger amplification during future seismic events is highlighted for a major part of the study area which comes under site class D and E. Also, the mismatch of site classes based on N30 and V30 s raises the question of the suitability of the NEHRP classification system for the study region. Further, 17 sets of SPT and SWV data are used to develop a correlation between N-SPT and SWV. This represents a first attempt of seismic site classification and correlation between N-SPT and SWV in the Indo-Gangetic Basin. Key words: Subsurface lithology, site classification, deep basin, NEHRP, MASW, SPT.
1 Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India. E-mail: anbazhagan2005@ gmail.com;
[email protected];
[email protected]
1. Introduction During an earthquake, local geology plays a significant role in controlling the surface effects of earthquakes. Subsurface soil properties can change the amplitude, frequency, and duration of bedrock motion when it reaches the surface. This phenomenon is known as the site effect and can cause additional damage during an earthquake in the form of excessive ground shaking, liquefaction, and landslides. Numbers of examples are available in the literature where local soil led to more damage during earthquakes in India (RAJENDRAN et al., 2003; AMBRASEYS and BILHAM, 2003; KHAN, 1874; OLDHAM, 1883; MAHAJAN and VIRDI, 2001; BENDICK et al., 2001; NIHON, 2011). A thick soil deposit with high water table is at heavy risk of undergoing excessive settlement and liquefaction during an earthquake. Before assessing these hazards, it is necessary to understand the geology of the region by focusing on subsurface soil characteristics and stratification details. This will help to understand the local site effects and induced effects accurately, so that such effects during any future earthquake can be precisely forecasted. The roles of locally available soil in modifying seismic waves are inevitably reflected in modern seismic codes. The determination of appropriate elastic response/design spectra according to soil categories and seismicity/site classification based on SPT-N and SWV is the simplest way to account for site effects in both engineering projects and for general purposes such as microzonation studies (PITILAKIS, 2004). It is also relevant to remember that these kinds of parameters concern only the elastic behavior of soils. Thus, for a more complete microzonation study, it is indispensable to perform regular tests on extracted soils. However, limited attempts have been conducted for
